Electrical coils for generating magnetic fields are widely used for, inter alia, influencing the path of an electron beam in an electron beam lithography machine, an electron microscope or other equipment generating and deploying electron beams for various purposes. Thus, for example, deflector coils are used in lithography machines to provided controlled deflection of a beam with the object of writing a pattern, such as an integrated circuit format, on a substrate. The quasi-continuous operation of such a coil results in ohmic heating of the coil wire and consequently an appreciable output of heat. Known coils are conventionally carried by formers of plastics material, which is a relatively poor conductor of heat and leads to considerable retention of heat by the wire. The resulting increase in wire temperature causes expansion of the coil, especially if multiply wound, and other stresses in the former and thus distortion of the magnetic field. The distortion detracts from accuracy of beam spot placement on the substrate and the reduced accuracy in turn causes increased tolerances in the pattern written by the beam. The finer the pattern features, the greater the effect on writing accuracy by beam positional error attributable to the thermal loading of the coil, in particular the former supporting the coil.